1. Field of the Invention
The present invention relates to a method of painting an elongated workpiece mainly having a complicated cross-sectional shape such as a door sash of an automobile.
2. Description of Related Art
There have hitherto been known methods of painting elongated workpieces, as disclosed in Japanese Published Unexamined Utility Model Registration Application No. 69580/1988 and in Japanese Published Unexamined Utility Model Registration Application No. 123555/1991. In the above-described methods there are used painting guns, each of which is made up of a ring-like gun main body having an opening or slit in one circumferential portion or a circumferentially discontinued portion (hereinafter called a circumferential opening) through which the elongated workpiece as an object to be painted can pass, and a plurality of nozzles which are radially disposed on the gun main body to eject a paint towards the central portion of the gun main body. The elongated workpiece is painted by moving the painting gun in the longitudinal direction of the elongated workpiece in a condition in which the elongated workpiece is placed in position inside the painting gun by passing it through the circumferential opening.
In the above-described conventional methods of painting, a uniform painting can be made if the elongated workpiece has a simple cross-sectional shape such as a circle. However, in painting a workpiece of a complicated cross-sectional shape such as a door sash of an automobile, there is a disadvantage in that the amount of paint reaching the inside of a recess such as a groove formed on the surface of the workpiece is insufficient, giving rise to a so-called phenomenon of lack of hiding. If, on the other hand, an amount of the paint to be ejected is increased in an attempt to cause the paint to reach the inside of the recess, another disadvantage may occur in that an excessive amount of paint is adhered to other portions of the workpiece, giving rise to a so-called phenomenon of sagging or run.
As another method of painting an elongated workpiece having a complicated cross-sectional shape, there is known the following method as disclosed, for example, in Japanese Published Unexamined Patent Application No. 180768/1987. Namely, there is provided a painting booth which is filled therein with an atomized paint. The paint is thus caused to be uniformly adhered or painted to the entire surface, inclusive of a recess, of an elongated workpiece by passing the workpiece through the painting booth. However, in the case of the door sash of the automobile as described above, the distance or clearance between the sash and the automobile body is relatively small. Therefore, it is impossible to provide a painting booth of a size large enough to enclose the door sash. It follows that this kind of painting method using the painting booth cannot be applied.
In view of the above disadvantages, the present invention has an object of providing a method of painting an elongated workpiece having a complicated cross-sectional shape in which method the workpiece can be uniformly painted by a ring-type painting gun without using a painting booth.